The present invention relates to a filtering device and method for filtering out supply voltage interference of a voltage source which delivers a supply voltage for an electrical load, having a capacitor which establishes a connection between the voltage source and the load to a reference potential.
A filtering element having a coil and a capacitor and referred to as an LC filter may be used for suppressing line-conducted system interference. It should ensure that no interference, for example, of a motor vehicle generator or another vehicle component can be heard in an audio signal of a car radio, for example. In order to protect a load to be connected from damage due to accidental polarity reversal, a power diode may also be provided for tripping a fuse in the event of polarity reversal.
The aforementioned LC filter is, however, an oscillating system whose resonance frequency is in the range of a bass frequency of audio reproduction. In the event of resonance, the energy stored in the capacitor""s coil is moved back and forth between these two components and thus may not be transmitted to the load, i.e., an LF power output stage of a car radio or a stereo system, for example. As a result, the output stages of car radios, for example, may not be supplied with sufficient bass frequency current, resulting in an increased distortion factor in the bass range, which is referred to as xe2x80x9cbass bulge.xe2x80x9d
It is believed that another disadvantage of the use of such an LC filter is that the effect of the coil in the case of high load currents, i.e., in the case of a high audio output power of a car radio as a load, for example, may diminish considerably, so that in this case only an ohmic resistance and a highly reduced reactance of the coil remain effective. In this case, an AC current load on the capacitor and thus its power loss may become excessive, resulting in accelerated aging of the capacitor. The magnitude of the load depends on the distribution of the current flowing through the coil resistance and the internal resistance of the capacitor. Accordingly, if the internal resistance of the coil is low, the load on the capacitor is similarly low.
An object of an exemplary embodiment and/or exemplary method of the present invention is to provide an improved filtering device of the above-named type and a corresponding method for operating it, which would eliminate the above-named disadvantages and achieve improved filtering of interference while affecting the load as little as possible.
It is believed that this objective may be achieved with a filtering device of the above-named type, in which a diode is connected between the voltage source and the capacitor, and to which a variable resistor is connected in parallel.
It is believed that this has the advantage that a coil-less filter is made available, which eliminates the disadvantages introduced by the coil in the systems referred to above, while its operation is not affected and is therefore improved, since there is no oscillating system.
An aspect of an exemplary embodiment of the present invention is that the coil is replaced in the filtering element by a diode, which is bridged by a controlled resistor in the event of high load currents. At the same time, the polarity reversal protection diode may be omitted, so that, in the event of accidental polarity reversal when the load is connected, the fuse is not destroyed. Since the circuit according to the exemplary embodiment of the present invention is not an oscillating system, it is believed that all disadvantages associated with such systems may be eliminated, since there is no more resonance or bass bulge.
Since the capacitor cannot be discharged via the power supply or the vehicle electrical system, which may occur in the case of an LC filter, it is believed that the undesirable phenomena associated therewith such as clicking noise due to an audio output stage may be avoided.
In an exemplary embodiment, the variable resistor is an electrically controllable resistor, the diode and the resistor being arranged as a MOSFET transistor connected between the voltage source and the capacitor, which is provided with a control circuit for controlling the variable resistor.
In order to ensure sufficient power supply to the load in the event of high power demand, the control circuit for deriving a control signal for the variable resistor is connected to a power output of the load, for example.
The voltage source may be a motor vehicle electrical system and the load may be an automotive audio component, a car radio in particular. The reference potential may be a ground potential, for example.
In operating the exemplary filtering device according to the exemplary embodiment of the present invention, the resistance of the variable resistor is reduced as the power consumption of the load increases.